As is known, apparatus for growing hollow, tubular crystalline bodies of the type disclosed in U.S. Pat. No. 4,544,528 comprise a crucible for supporting molten silicon which constitutes the feed material from which the crystalline body is grown. In the past, the silicon melt was replenished in batches rather than continuously so as to avoid thermally shocking the system, thereby catastrophically terminating the crystal-growing process. Such batch replenishment was found to be a relatively slow and inefficient method of adding silicon particles to the melt.
To overcome the problems associated with batch replenishment, a system was developed for continuously replenishing the melt. This system is described in U.S. Pat. No. 4,661,324, issued Apr. 28, 1987 in the name of Sink et al. (the 324' patent). The system of the 324' patent includes (1) a conduit leading up through the crucible and terminating in the interior of the inner after-heater of the furnace above the tip of the die and (2) a chip thruster coupled to the conduit and to a source of solid irregularly-shaped silicon particles. Silicon particles are fed into the chip thruster which periodically forces a predetermined volume of the particles up through the conduit and into the interior of the inner after-heater. Thereafter, the chips fall, under the pull of gravity, into the melt.
Unfortunately, the system of the 324' patent suffers from several problems. First, because the chip thruster portion of the system includes a significant number of mechanical parts, and because the chip thruster is constantly handling abrasive material (i.e. irregularly-shaped particles of solid silicon), the chip thruster tends to break down. Second, the abrasive silicon particles tend to erode or abrade away minute portions of the metallic components of the chip thruster, which portions are delivered along with the silicon particles into the melt. These minute portions of metal tend to contaminate the melt. Third, because the mass of a predetermined volume of irregularly-shaped silicon particles varies from one volume of particles to the next depending upon how the chips orient themselves relative to one another, and because the chip thruster ejects a predetermined volume, not mass, of particles into the furnace, it is virtually impossible to repeatedly add a predetermined mass of chips to the furnace. To minimize thermal excursions to the level necessary to maintain a substantially continuous crystal growth process, it is important where irregularly-shaped silicon particles are used as the feed material that a predetermined mass of particles always be added to the melt. For the reasons noted above, the chip thruster system is not capable of repeatedly providing such a predetermined mass of irregularly-shaped particles to the melt.